Hitherto, very little literature has been published on the surface modification of polyisocyanates which are solid at room temperature.
German Offenlegungsschrift No. 2,557,407 describes a process in which a solution of a polyisocyanate in a low-boiling solvent is sprayed into a reactor with gaseous di- and/or polyamine. Hollow beads of polyurethane polyurea (which are preferably used as fillers) are obtained by the reaction of the polyisocyanate with the amine and by evaporation of the solvent. The reaction is generally conducted in such a way that the NCO-groups react off completely with the amine and any other NCO-reactive components (for example diols) added.
U.S. Pat. No. 3,409,461 describes the coating of polyisocyanates with a protective substance, preferably a polymer, to deactivate the polyisocyanate particles at their surface. The isocyanate is dispersed in a solution of the polymer in a low-boiling solvent which has very little dissolving effect on the isocyanate. The dispersion is then spray-dried. Finely ground (particle size 1 to 10 .mu.m) naphthylene-1,5-diisocyanate is preferably spray-dried with a 1 to 2.5% solution of polystyrene, polyvinyl butyl ether, chlorinated rubber and the like in tetrachloromethane. Free-flowing powders having particle sizes of from about 1 to 50 .mu.m are obtained. These powders are suitable for improving the adhesion of polyester products (woven fabrics, fibers, films) to rubber elastomers. In this process for coating isocyanates with added polymers from solution, considerable quantities of solvents (which may be toxic) have to be used (for example 4 kg of tetrachloromethane for 50 g of naphthylene-1,5-diisocyanate) and then removed again in an energy-consuming operation. One particular disadvantage of the process lies in the high percentage of coating material (from 9 to 91% by weight; in the Examples, it is generally of the order of 50% by weight) in the total weight of the coated isocyanate. As a result of this, an excessive proportion of troublesome foreign substance would have to be introduced in the production of high-quality polyurethanes.
U.S. Pat. No. 3,551,346 describes the encapsulation of liquid diisocyanates by interfacial reactions of CH.sub.3 --Si--(OCH.sub.3).sub.3 dissolved in the diisocyanate with (CH.sub.3).sub.3.Si--O--Na dissolved in the aqueous phase to form a film. These droplets preencapsulated by silicone polymer formation are then "encapsulated" by coacervation (for example with oppositely charged polymers in accordance with U.S. Pat. No. 2,800,457).
German Offenlegungsschrift No. 2,311,712 describes a process for the encapsulation of solid substances by a polymer shell formed from NCO-prepolymers and chain-extending agents. The reactive mixture and an aqueous phase are introduced into a zone of high turbulence at a temperature at which all the reactants are liquid. Microcapsules are formed through the formation of a high moleculare weight polymer as the coating material (for example a polyurea of NCO-prepolymers and polyamines). This known process may be used to encapsulate any solids or liquids which are inert to the NCO-prepolymers and the chain-extending agents (and water) and are insoluble in water (for example tris-chloroethyl phosphate flameproofing agents, plasticizers, fragrances, etc.). Similar processes are described, for example, in U.S. Pat. No. 4,120,518 for encapsulation reactions with carbodiimide-containing polyisocyanates to form the filling.
German Offenlegungsschrift No. 1,570,548 describes a one-component system of relatively long shelf-life consisting of a mixture of (i) 1 moles of a polyester, polyether or polythioether, (ii) at least 1.5 moles of a solid isocyanate containing uret dione groups and having a melting point of 100.degree. C. or more and (iii) at least 0.3 mole of a solid chain-extending agent containing OH- and/or NH.sub.2 -groups and having a melting point of 80.degree. C. or more. At least 80% of the solid constituents of the mixture are required to have a particle size of 30 .mu.m or less. The shelf life of this one-component system amounts to between a few days and a few weeks at room temperature, but only to a few hours at 50.degree. C. One disadvantage of this known process is that at least two of the three reactants have to be present in the solid form to guarantee the requisite shelf life. The effect of this is that the mixtures obtained generally have very high viscosities and their viscosities continue to increase slowly because none of the compounds has been adequately modified in its reactivity. The reaction at the surface of the solid particles, which is reflected in the steady increase in viscosity, takes place uncontrolled and too slowly in practice and does not sufficiently retard the reactivity of the polyisocyanates to the point where the system is self-stabilizing. In addition, when the mixture is hardened, inhomogeneities are inevitable in the fully heated product due to the high percentages of solid constituents. Processing of the highly viscous to solid mixtures is also more difficult because, in contrast to liquid mixtures, they first have to be brought into a formable condition either by increasing temperature or by applying pressure.
Comparative tests have shown that, when high-melting polyisocyanates are mixed with relatively high molecular weight and low molecular weight polyols, a constant and relatively rapid reaction takes place with a marked increase in viscosity. In other words, the surface reaction on the solid polyisocyanate particles does not form a coating around the polyisocyanate which is sufficient for retarding, i.e. has an adequate stabilizing effect.
British Patent No. 1,134,285 describes a process for the production of dimeric diisocyanates in an aqueous reaction medium. According to this reference, dimeric diisocyanates produced in this way do not react with polyfunctional compounds containing reactive hydrogen atoms at room temperature, although mixtures with polyols may be thermally crosslinked to form polyurethanes. Stability may possibly be brought about by a slow surface reaction of isocyanate groups with water. Crosslinking is then brough about at high temperatures, for example 150.degree. to 200.degree. C., by cleavage of the uret dione ring.
Storable reactive systems having long pot lives are described in German Offenlegungsschriften Nos. 2,842,805 and 2,941,051. According to these references, polyisocyanate mixtures of high-melting polyisocyanates (for example dimeric TDI) and liquid polyisocyanates are mixed with relatively high molecular weight and low molecular weight polyols, optionally in the presence of subequivalent quantities of a compound containing from 2 to 4 amino groups (for example cycloaliphatic diamines or triamines, hydrazine and subsituted hydrazines or acid hydrazides), to form a dispersion-stable system which, in a second stage, is hardened in molds at temperatures above 90.degree. C., optionally after the addition of glass fibers.
Polyisocyanates stabilized with subequivalent quantities (0.01 to 20 or 25% of the NCO-groups present) of diamines and other NH-functional compounds in admixture with polyols, polyamines or polyhydrazides are also described in German Offenlegungsschriften Nos. 3,112,054 (which corresponds to U.S. Pat. No. 4,400,497). 3,228,723, 3,228,724, 3,228,670 and 3,230,757 (which corresponds to U.S. Pat. No. 4,483,974). The polyisocyanate particles are thus deactivated at their surface with up to 25 equivalent percent of all the NCO-groups present to form a polymer (for example polyurea) coating. Dispersions of deactivated polyisocyanate particles such as these in polyols and/or polyamines show at least very good stability in storage at room temperature. It is only above a solidification or thickening temperature that the reactive system enters into polyurethane(urea)-forming reactions and, in that case, forms elastomers or similar end products.
As already observed in German Offenlegungsschrift No. 3,230,757, the compounds used for the surface reaction (for example the aliphatic polyamines), generally do not react off completely--largely irrespective of the quantity of amine--if they are reacted in dispersion in polyols or polyamines. Thus, part of these polyamines remains unreacted in the reactive dispersion. This can occasionally produce a desirable effect insofar as any surface defects in the polyurea coating which may begin to form during storage or handling of the reactive mixture are cured again ("self-healing effect") so that stability in storage remains unaffected.
On the other hand, however, this effect involves a serious disadvantage where it is attempted to produce relatively large (large-volume) moldings. This is because it has been found that, during the slow heating of reactive systems (due to the poor transfer of heat in the molding or in the event of prolonged heating to temperatures just below the thickening point of the system, which can often be necessary to improve the pourability of the systems) or after partial solidification on the hot outer mold, the still liquid reactive dispersion core containing unreacted amine stabilizer is further deactivated so that the thickening temperature in the core continues to increase. This gives rise to the formation of inhomogeneous bubbles. In addition, the liquid core can break the skin open and leak out, thus damaging the surface. Accordingly, the strength of inhomogeneous moldings such as these is unsatisfactory.
Another disadvantage of the known stabilizing process lies in the fact that, if the unreacted stabilizer component remaining in the dispersion is removed, the stability of the dispersions in storage is often distinctly reduced or even lost altogether.
For this reason, it is not possible to stabilize a polyisocyanate in isocyanate-reactive compounds (for example polyols) in accordance with the prior art by surface modification of the polyisocyanate particles with a stabilizing component (for example an aliphatic diamine), and then to carry out reactions between the isocyanate-reactive compounds and standard liquid polyisocyanates in order to obtain preextension of a long-chain polyol which may be necessary to obviate viscosity problems or to obtain some degree of thixotropy. In most cases, the liquid polyisocyanate added normally destabilizes the dispersion.
In addition, where aliphatic diamines are used as the stabilizing component ("amine stabilizer"), the addition of compounds which are reactive to, or absorb, aliphatic diamines (such as for example halogen-containing flameproofing agents and blowing agents, zeolites, fatty acids, phosphoric acid esters, and certain solvents) is not recommended and, in some cases, is impossible. The same also applies to deactivating agents of other chemical types.
According to the references mentioned above, stabilization of the polyisocyanate by reacting the isocyanate particles with a stabilizing component may even be carried out before mixing with the isocyanatereactive compounds by reacting the polyisocyanate with the stabilizing component in an inert solvent which does not dissolve the polyisocyanate, followed by isolation, so that no unreacted "amine stabilizer" component is present in the polyisocyanate. However, this brings another disadvantage of the known process. Because no excess deactivating component is present where this solid polyisocyanate deactivated by a preliminary reaction is used, the dispersions containing isocyanate-reactive compounds prepared with the solid deactivated polyisocyanate are no longer self-healing and are highly susceptible to mechanical and/or thermal influences.
Another disadvantage of the known stabilizing process is that up to 25 equivalent percent of the solid (generally very expensive) polyisocyanate component is used up during stabilization and is no longer available for further polyurethane reactions.